1. Field of the Invention
The present invention relates to a beam projecting device for forming a reference plane by projecting a light beam, such as a laser beam, while rotating a light beam projecting portion of the beam projecting device.
2. Description of Related Art
A known example of the type of device mentioned above is a laser surveying device or so called "laser planer". In this device a rotating member projects a laser beam emitted by a laser light source in, approximately, a horizontal direction toward an object to be surveyed, which is then scanned so as to form a reference plane. In this case a height of a laser beam spot against the object to be surveyed can be either visually observed through the eye or through the use of a detector. This type of laser surveying device is known (refer to Unexamined Japanese Patent Publication No. 5-322564).
Known laser surveying devices are mostly used for forming a reference plane in a wide distance range, from a short distance (e.g. 0.5 m-1.5 m) to a long distance (e.g. 100 m-200 m). In this case, simple focusing is desired, and detection sensitivity should be prevented from varying when a beam position is detected by a detector. Also it is desirable that a beam detector can precisely detect a position of a laser beam, emitted by a laser surveying device, and that the projected laser beam is clearly visible to the naked eye.
Since laser surveying devices are often used outdoors in extreme conditions (e.g. -20.degree. C. to +50.degree. C.) the focal point may deviate due to an expansion or contraction of a lens and/or a lens supporting frame caused by a variation in temperature.
Normally, variations in temperature cause the beam waist position to shift due to an expansion or contraction of the lens supporting frame, variations in the refractive index of a lens itself, the variation of an oscillation wavelength of a laser light source, etc.
FIGS. 68 and 69 are graphs showing the relationship between a projecting distance of a laser beam and a beam diameter at a constant temperature. In FIG. 68, the initial projecting beam diameter W is 8 mm. In FIG. 69, the initial projecting beam diameter W is 12.5 mm. In both cases, the wavelength of laser beams is the same, and symbol "x" in each figure indicates a beam waist position.
It is understood that when the beam waist position x varies, the beam diameter varies at each distance in both figures. For example, in FIG. 69, when the laser surveying device is used at a close distance range, ranging from about 10 m to 70 m, the beam waist position x should be set at 50.879 m. On the other hand, when the device is used for a long distance range, ranging from about 100 m to 200 m, the beam waist position x should be set at 97.505 m.
FIGS. 66 and 67 are graphs showing variations in beam diameters at different projecting distances while using the variation in temperature as a parameter. In these figures the diameter of a laser beam at different projecting distances is plotted under the condition that a laser surveying device is provided with a lens system for making a parallel beam of an adequate size and where the diameter of the laser beam at the projecting end of the device (i.e. initial projecting beam diameter), and a predetermined beam waist position X (i.e. a distance from the projecting end of the device to a position where the beam waist is formed) are given. In FIGS. 66 and 67, the initial projecting beam diameters W are 8 mm and 12.5 mm, respectively.
FIG. 66 shows that when the temperature is varied from -20.degree. C. to +50.degree. C., the variation of the diameter of a laser beam at different projecting distances for a laser surveying device having a projecting beam wavefront with a radius of curvature R of 78 m, under the condition that the beam waist position X is initially set at 39.578 m at a temperature of 20.degree. C., that is, the distance from the projecting end of the device to the position at which the beam waist is formed is 39.578 m.
Likewise, FIG. 67 shows that when the temperature is varied from -20.degree. C. to +50.degree. C., the variation of the diameter of a laser beam at different projecting distances for a laser surveying device having a projecting beam wave front with a radius of curvature R of 190 m, under the condition that the beam waist position X is initially set at 96.605 m at a temperature of 20.degree. C., that is, the distance from the projecting end of the device to the position at which the beam waist is formed is 96.605 m.
As can be understood from FIGS. 66 and 67 even if an initial projecting beam diameter, a beam waist diameter, and a beam waist position are adequately determined initially, the beam waist position X varies due to variations in temperature and the beam diameters are not equal at each beam waist position X at different temperatures.
When a semiconductor laser is used as a light source, since it provides a diverging laser beam, a collimating lens is used to obtain an approximately parallel laser beam. Transverse magnification of the optical system of the laser projecting device is given as the ratio of the numerical aperture of an incident side of a laser beam from the semiconductor laser, to the numerical aperture of an outgoing side thereof. When it is desired to effectively utilize a laser beam of the semiconductor laser, since the numerical aperture of the incident side of the collimating lens should be approximately 0.2-0.4, and the numerical aperture of the emitting side of the collimating lens should be approximately 0.00002-0.0005, a magnifying and projecting system with a very large magnifying power thus needs to be constructed.
Since a laser beam with the above mentioned large magnifying power is projected from the laser surveying device, a slight variation to the laser beam in the laser surveying device causes quite a large variation to the laser beam when it is projected from the laser surveying device. This variation increases over distance. Therefore, in a conventional laser surveying device, the following problems may occur: a converging point is greatly deviated from a designed position; the beam diameter becomes too large at a distance far away from the device so as not to be precisely detected by the laser detector; and the luminance required for visual observation is insufficient, which causes the problem of only having a short usable distance, when a long distance may be required.
In order to solve the problem described above, in the U.S. Pat. No. 5,225,928 a lens has been proposed which has an overall index of refraction which changes with temperature and wavelength. This change in index of refrection changes the focal point of the lens for the laser beam in an amount which substantially compensates for defocusing caused by temperature effects on the mounting length and laser beam wavelength. However, optical glasses used in a collimator are not available to precisely compensate for all variations in temperature that may occur. Additionally, in the case when a semiconductor laser with a large radiation angle is used as a light source, a collimating lens with a large numerical aperture (NA) is required, this makes it much more difficult to completely compensate for the variations in temperature.
However, since the variation in oscillation wavelength is not always the same in all semiconductor lasers, and a laser surveying device is designed according to a standard semiconductor laser, a desired performance may not be expected when the laser surveying device is provided with certain semiconductor lasers.
Previously, even when a laser surveying device was designed so as to completely compensate for variations in temperature, it was quite difficult in reality to completely eliminate the influence of temperature variation. The reason for this is because temperature distribution occurs in each member constituting the laser projecting device, for example, the laser projecting device includes a light source such as a semiconductor laser and the like, and the light source itself is a heat source which causes a difference in temperature between the device and the outside, which causes a variation in temperature. Further, heat generated by a motor used to rotate a laser beam projecting portion of the device cannot be neglected. Therefore, even if a laser surveying device was previously designed, in theory, so as to completely compensate for variations in outside temperature, a desired beam diameter at a prescribed distance could not be obtained in reality, where various types of temperature change occur.